1. Field of the Invention
The invention relates to a system for tensioning a riser extending between a floating offshore platform and a submerged riser base.
2. Description of the Prior Art
In the offshore oil and gas industry exploratory well drilling operations are generally carried out from floating drilling platforms. These platforms may consist of a ship-like hull or of a semi-submersible vessel in which a central hull opening or "moon-pool" is created through which a riser extends in a downward direction. The riser may consist of a large diameter pipe column which is at its lower end secured to a subsea wellhead assembly and which is at the upper end thereof connected to the platform by means of a riser tensioning system. The riser tensioning system generally comprises a cable and sheave mechanism that applies an upward force to the riser and that allows the floating platform to make oscillating motions relative to the stationary riser.
In deepwater areas where oil or gas is produced from submerged wells floating production platforms may be used to which one or more production risers are connected by riser tensioning systems similar to those used on drilling platforms.
The production riser may be secured at the lower end thereof to a wellhead or to another kind of submerged riser base located on or at a distance above the waterbottom.
If the oscillating movements of the platform due to wind, waves, and current become too rough, drilling or production operations have to be interrupted and the riser disconnected from the wellhead or riser base.
Under such circumstances it is a general practice to disconnect the riser from the riser base by activating a quick release coupling at the lower end of the riser and to subsequently allow the riser tensioning system to lift the disconnected riser quickly from the riser base thereby avoiding sweeping movements of the lower end of the riser that may cause damage to the subsea equipment at or near the riser base. Then, a short while after disconnection, the lifting force applied by the tensioning system to the riser is usually reduced by manual control to avoid increases in the vertical movement of the riser to such an extent that the upper end of the riser collides with the equipment arranged on the platform above the riser, such as the rotaty table.
An apparatus need be developed however that automatically compensates for the upward recoil motion of the riser when it is released from the wellhead.